Distilled water evaporates, as all liquid water does, but its rate of transition to a gaseous state differs slightly from tap water. This difference is not due to a separate physical process but is a consequence of the water’s chemical composition. Understanding this requires looking at how water changes phase and the impact of dissolved substances.
The Physical Process of Evaporation
Evaporation is the process where a liquid turns into a gas without reaching its boiling point. This transformation is a surface phenomenon, occurring only at the interface between the liquid and the air above it. Water molecules are constantly in motion, possessing a range of kinetic energies.
A molecule can only escape the liquid surface if it has enough kinetic energy to overcome the attractive forces, primarily hydrogen bonds, holding it to its neighbors. The collective pressure exerted by these escaped molecules in the air above the liquid is known as vapor pressure. A higher vapor pressure indicates a greater tendency for the liquid to evaporate.
The Composition of Distilled Water
Distilled water is a highly purified form of water that contains almost nothing but H2O molecules. It is produced through distillation, which involves boiling the water and then collecting the resulting steam. This steam is then cooled, allowing it to condense back into a liquid state.
This purification method effectively leaves behind nearly all non-volatile substances, including minerals, salts, and other dissolved solids or contaminants. Tap water, conversely, is a solution containing various amounts of these solutes, such as calcium, magnesium, and sodium ions.
How Purity Affects Evaporation Rate
The presence of dissolved solids in tap water slightly reduces its evaporation rate compared to distilled water. This effect is a colligative property, meaning it depends on the number of solute particles present, not their chemical identity. The dissolved particles interfere with the water molecules’ ability to escape into the air.
These solute particles occupy space at the liquid’s surface and exert attractive forces on the surrounding water molecules. This stabilizes the liquid phase, making it harder for water molecules to gain the necessary energy to break free. Because of this interference, tap water has a slightly lower saturation vapor pressure than distilled water under the same conditions.
The lower vapor pressure means fewer water molecules successfully transition to the gas phase. Therefore, distilled water, being virtually pure, has the highest possible vapor pressure for water and will evaporate marginally faster than tap water. While the difference is scientifically observable, it is often negligible in most everyday practical settings.
Universal Factors Influencing Water Evaporation
While composition plays a minor role, external conditions are the most significant drivers of the evaporation rate for any type of water. Temperature is the most influential factor, as increased heat directly raises the kinetic energy of the water molecules. This allows a greater number of them to overcome intermolecular forces and escape the liquid surface.
Surface area also plays a major role because evaporation is a surface phenomenon; a wider container exposes more liquid to the air, accelerating the process. The humidity of the surrounding air is also significant, as air that is already saturated with water vapor cannot accept much more, thus slowing evaporation. Air movement, such as wind, increases the rate by sweeping away the layer of water-saturated air near the surface, replacing it with drier air.